The present invention concerns coatings prepared from adducts of polyepoxides with polyamines and blocked polyisocyanates.
German No. 1,086,372 describes varnishes resistant to chemicals and solvents prepared from a polyamide (the reaction production of a dimer acid and a polyamine) and a trimethylolpropane/toluene diisocyanate prepolymer blocked with a phenol. Cure will take place at room temperature. Addition of an epoxy resin to the above blend results in improved adhesion (German No. 1,090,803). Flexibility of epoxy resins cured with polyamines at room temperature can be improved by conducting the cure in the presence of a TMP/TDI prepolymer capped with nonylphenol (Ger. Offen. DE No. 2,152,606).
In U.S. Pat. No. 3,321,548 resinous coatings with a fairly high degree of crosslinking and rigidity are obtained by reacting a phenol-blocked isocyanate with functionality of at least three with an amine terminated epoxy resin derived from an epoxy resin and an aromatic amine. Phenol-capped isocyanates are not acceptable in the present invention because they react with the amine-capped epoxy at room temperature and, therefore, do not have sufficient stability to formulate a one-package system.
U.S. Pat. No. 3,886,228 provides compositions from a polyamine terminated imide and an isocyanate or phenol blocked prepolymer. Again, these are room temperature curing and unacceptable as single-package systems.
Amine terminated polyamide systems stable at room temperature are provided by blocking the isocyanate prepolymer with an alcohol. These systems are cured above 350.degree. F. (177.degree. C.) (U.S. Pat. No. 2,995,531). Mixtures crosslinkable at elevated temperatures are obtained from a trimethylolpropane/toluene diisocyanate prepolymer capped with .epsilon.-caprolactam and cured with 3,3'-dimethyl 4,4'diaminodicyclohexyl methane (Ger. Offen. DE No. 2,131,299).
Chain extended, substantially linear elastomers prepared from a diamine and a ketoxime blocked diisocyanate are described in Canadian No. 788,481.
Electrodepositable compositions are described in U.S. Pat. Nos. 4,104,147 and 4,115,226. These are based on intermediate molecular weight amine terminated epoxies cured with blocked isocyanates, but these must be used in aqueous dispersions to provide suitable application viscosity. Amine terminated epoxies capped with an anhydride to give anodic electrodepositable coatings which may be cured with a blocked isocyanate are described in U.S. Pat. No. 4,176,221.
Automotive primer-surfacers are typically cured at 275.degree.-325.degree. F. (135.degree.-163.degree. C.). To be useful, such coatings must cure sufficiently within 15 minutes (900 s) at 275.degree. F. (135.degree. C.) to give suitable properties. Such a coating must pass a number of tests, one of the most important being a chip resistance test described in Society of Automotive Engineers (SAE) Test Method J400. A chip rating of 6-7 is generally acceptable by the automotive companies. Recent environmental pressures have required that newer coatings have a lower volatile content.
The present invention provides coating compositions with higher solids, i.e. non-volatiles content, which are capable of curing within about 15 minutes (900 s) at 275.degree. F. (135.degree. C.) and provide suitable chip resistance.